Spherical titanium dioxide particles and process of manufacture

ABSTRACT

Titanium dioxide, in the form of discrete, solid substantially spherical particles of a substantially uniform shape and size, is obtained by providing an aerosol comprising discrete liquid particles of a hydrolyzable titanium compound, contacting the aerosol with water vapor in dynamic flow to hydrolyze the liquid titanium (IV) compound to titanium dioxide, and recovering the desired product.

FIELD OF THE INVENTION

This invention relates to a process for the preparation of titaniumdioxide and to the resulting product. More particularly, the process isbased on the controlled hydrolysis of a liquid aerosol comprising avolatile liquid titanium (IV) compound. The process results in theformation of titanium dioxide in the form of substantiallynon-aggregated solid particles having a uniform spherical shape, and ina narrow size distribution.

BACKGROUND OF THE INVENTION

Titanium dioxide has a wide variety of uses, e.g., as a pigment, acatalyst or a photoconductor, as well as other uses. Processes for thepreparation of titanium dioxide are known in the art. In one suchprocess, the ore ilmenite, containing titanium and iron, is treated withsulfuric acid, and the resulting solution is thermally hydrolyzed andthen calcined in the presence of salts and/or orienting nuclei. Anothersuch process is based on the chlorination of mineral rutile and/orenriched titanium-containing ores, to form titanium tetrachloride,followed by the purification of the titanium tetrachloride and itsoxidation in the presence of other chlorides, especially AlCl₃.

The titanium dioxide provided by such processes typically is in the formof irregularly shaped prismatic or spheroidal particles, also having, ingeneral, a broad size distribution. As is known, a broad granulometricdistribution of titanium dioxide particles detracts from the usefulnessof this material in many commercial applications. For instance, thecolor purity and optical performance of titanium dioxide pigments,either alone or in admixture with other pigments, may be adverselyaffected. Moreover, the titanium dioxide particles from these processesare often aggregated, and as a rule long and expensive procedures forbreaking down the aggregates into smaller, individual particles arerequired.

On the other hand, it is known that the hydrolysis of an aqueoussolution of titanium tetrachloride, leads to the formation of titaniumdioxide as needle-shaped particles having a predominantly rutile crystalstructure.

E. Matijevic et al, in the Journal of Colloid and Interface Science,Vol. 61, page 302 (1977), describe the preparation of non-aggregatedspherical particles of titanium dioxide, in a narrow size distribution,by the hydrolysis of a solution of titanium tetrachloride at elevatedtemperatures, in the presence of sulfuric acid (sulfate ions). Themethod is not entirely satisfactory, however, because very longprocessing times are required, yields are low and very small particles,e.g., about 0.2 μm or less, which are useful in pigment manufacture, arenot obtained. Moreover, not all of the titanium dioxide particles appearas regular spheres.

It is desirable that titanium dioxide particles have the followingcharacteristics:

(i) a narrow size distribution;

(ii) the substantial absence of aggregation; and

(iii) a substantially uniform spherical shape.

These characteristics permit the application of rigorous lightscattering principles to the optical behaviour of the particles, and,thus, to determine in advance the optimum diameter of the titaniumdioxide for its various uses.

OBJECTS OF THE INVENTION

It is an object of this invention to provide titanium dioxide indiscrete particulate form, in a narrow particle size distribution, theparticles also having a substantially uniform spherical shape.

It is another object of this invention to provide a process for theformation of spherical particles of titanium dioxide which permits awide selection of particle diameters, the desired diameter beingobtained within a narrow size distribution range.

These objects are realized by the invention herein described.

DESCRIPTION OF THE INVENTION

In its broadest aspects, this invention provides a method of preparingtitanium dioxide, comprising:

(A) providing a liquid aerosol comprising discrete liquid particles of ahydrolyzable titanium (IV) compound;

(B) contacting the liquid aerosol with water vapor in dynamic flow, tohydrolyze the liquid titanium (IV) compound to titanium dioxide in theform of discrete, solid substantially spherical particles of asubstantially uniform shape and size; and

(C) recovering the titanium dioxide.

The titanium dioxide of this invention is recovered in the form ofsubstantially uniform spherical particles, substantially non-aggregated,having an average (modal) diameter in the range of from about 0.05 toabout 3 μm. The process permits the formation of titanium dioxideparticles of the desired average (modal) diameter in a very narrow sizedistribution. For instance, the width of particle size distribution, σo,may be as low as 0.1σo is the measure of width of the size distributionas defined in the paper by W. F. Espenscheid et al J. Phys. Chem., Vol.68, page 3093 (1964). In general, lower values of σo are indicative ofgreater uniformity of particle size.

In addition, the titanium dioxide particles of this invention can beprepared with different degrees of hydration. When anhydrous, theparticles are extremely pure, i.e., containing in excess of 99.8% byweight of titanium dioxide. The resulting particles are readilydispersible in water without lossess in the uniformity of shape andsize.

The titanium compounds used as starting materials in the process of thisinvention can be selected from a wide variety of hydrolyzable, volatile,liquid titanium compounds, preferably having a vapor pressure of about 1torr (1 torr=1 mm Hg) below about 200° C. By way of illustration, theseinclude titanium tetrachloride, and titanium (IV) alkoxides, preferablyof from 1 to 6 carbon atoms, e.g., titanium isopropoxide, titaniumethoxide, titanium pentoxide, or the like. The titanium tetrachloridecan be obtained conventionally by the chlorination of a titanium-bearingore, followed in the normal manner by purification and distillation. Inpractice, it is possible to employ titanium tetrachloride derived fromthe preparation of titanium dioxide by the well-known chlorinationprocess, referred to above.

The aerosol, comprising liquid droplets of the hydrolyzable titaniumcompound suspended in a carrier gas, is prepared using known procedures,e.g., nebulization. Preferably, in order to provide an aerosol havingdroplets of a very narrow size distribution, and a smaller modal size, afalling liquid film aerosol generator is used. In general, in such adevice, the liquid component is evaporated in an enclosed chamber, mixedwith a flowing carrier gas and subsequently condensed in the carrier gasat a lower temperature. If desired, the condensation can be carried outin the presence of heterogenous nuclei, which usually results in a morenarrow particle size distribution.

A preferred procedure for the preparation of the liquid aerosolcomprises the following steps:

(a) nucleation,

(b) evaporation,

(c) condensation,

(d) reheating, and

(e) recondensation.

The term "nucleation" is used herein in the conventional sense to referto the formation of a new phase from a homogeneous environment, such asa solid phase from a vapor phase or liquid phase from a vapor phase,under conditions of supersaturation. Nucleation may be controlled tooccur homogeneously or heterogeneously. Homogeneous nucleation takesplace when the new phase is self-induced, i.e., spontaneous at thecritical degree of supersaturation. Heterogeneous nucleation takes placewhen the formation of the new phase is induced by the presence of aforeign material, e.g., solid particles.

By way of illustration, in homogeneous nucleation the hydrolyzabletitanium compound, previously vaporized, is condensed, by cooling, intodiscrete liquid particles, in a flowing carrier gas. The carrier gas ispreferably an inert material, such as nitrogen, helium, air, preferablydried air, or any of the other gases conventionally employed as carriersfor solid or liquid aerosols. The procedure is carried out underconditions of relatively high supersaturation, i.e., the vaporizedtitanium compound is present in amounts of 200-300% over saturation.

When heterogeneous nucleation is applied, solid nuclei which can beproduced from a wide variety of materials, such as ionic, metallic andthe like, are introduced into a flowing carrier gas, such as any of theabove. The nature of the solid is not critical, and virtually any solidmaterial can be used. The solid nuclei are generated by first directingthe carrier gas over the solid, which has been heated to a temperatureat least sufficient to cause its sublimation or vaporization (which canvary widely with the material, as is illustrated in detail herein). Thevaporized solid is then cooled to below its vaporization or liquefactiontemperature and thus condensed into discrete solid particles, e.g. inthe range of from 10 to 100 A in diameter, in the flowing carrier gas.These solid particles act as condensation sites (nuclei) for thehydrolyzable titanium compound, which has been previously vaporized, andwhich is now admixed with the solid nuclei in the carrier gas. Incomparison with homogeneous nucleation, a smaller degree ofsupersaturation, e.g., 5% over saturation, is sufficient for the vaporsof the titanium compound to condense.

The technique of nucleation, both homogeneous and heterogeneous, is wellknown in the art, and is illustrated in greater detail below.

In comparison with heterogeneous nucleation, homogeneous nucleation ismore sensitive to temperature and pressure gradients in a flow system,which results in concentration gradients. Moreover, homogeneousnucleation is more easily affected by contaminants and irregularities incontainer surfaces. As a consequence, it is more difficult to controlthe particle size in a homogeneously nucleated system. Therefore,heterogeneous nucleation is preferred in carrying out the process ofthis invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a preferred process according to thisinvention.

FIG. 2 illustrates a preferred modification for the falling liquid filmaerosol generator shown in FIG. 1.

FIG. 3 represents a scanning electron micrograph (SEM) of sphericaltitanium dioxide particles according to this invention, obtained by thecontrolled hydrolysis of a titanium (IV) ethoxide liquid aerosol. Themagnification is 5000×.

FIG. 4 represents a scanning electron micrograph of the same sample asin FIG. 3 taken at a magnification of 1000×.

FIG. 5 represents a transmission electron micrograph of sphericaltitanium dioxide particles according to this invention, obtained by theconrolled hydrolysis of a titanium (IV) ethoxide liquid aerosol. Themagnification is 20000×.

FIG. 6 represents a scanning electron micrograph of spherical titaniumdioxide particles according to this invention, obtained by thecontrolled hydrolysis of a titanium (IV) isopropoxide liquid aerosol.The magnification is 5000×.

FIG. 7 is a graph showing the effect of the carrier gas flow rate andthe nuclei generator temperature on the particle size, in aheterogeneously nucleated system.

FIG. 8 is a graph showing the effect of the boiling temperature, for theliquid from which the liquid aerosol is made, on the particle size ofthe condensed liquid particles.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

With reference to FIG. 1, inert gas stream 2, passes through dryingcolumns 4 and 6, which are packed with a suitable drying agent, e.g.,magnesium perchlorate, phosphorus pentoxide, or the like. Dried inertgas stream 8, is filtered through a suitable filter 10. Filtered inertgas stream 12, is introduced, at a flow rate controlled by flowmeter 14,into solid nuclei generator 16, at a uniform flow rate.

Nuclei generator 16, contains sillimanite tube 18, in which is centereda "Vicor" glass boat containing a solid nucleating material, e.g.,silver chloride. The temperature of nuclei generator 16, is adjusted togive a suitable vapor pressure for the solid nucleating materialformation. This varies depending on the material used to form the solidnuclei. By way of illustration, in preparing octanoic acid aerosols,using AgCl as the nucleating material, a temperature of between 404° and645° C. in the nuclei generator is preferred. In the case of linolenicacid aerosols, prepared using AgCl or NaCl as the nucleating material,nuclei generator temperatures of 500°, 642° or 718° C. are preferred. Inthe case of dibutylphthalate aerosols, using NaCl as the nucleatingmaterial, nuclei generator temperatures of 590°, 645° and 690° C. areemployed. In the case of NaCl aerosols, using NaF as the nucleatingmaterial, a temperature of 850° C. is preferred. Suitable temperaturesfor other materials are readily determinable with only a small amount ofwork, or can be obtained by reference to the literature.

Inert gas stream 12 passes through sillimanite tube 18, and admixes withthe vaporized solid nucleating material. By way of illustration, in thecase of titanium (IV) alkoxides, the inert gas flow rate is maintainedbetween 150 and 1,500 ml/min., and for titanium tetrachloride between 40and 3,000 ml/min, but other flow rates can be used as long as theReynolds number is less than 2,000. Upon emerging from nuclei generator16, the vaporized nucleating material cools and condenses, to providestream 20, which comprises particles of solid nuclei suspended in theinert gas. Stream 20 is then thermally equilibrated in coil condenser22, to the temperature of the liquid aerosol generator.

Preheated stream 24, comprising the inert gas and solid condensationnuclei, passes through falling liquid film aerosol generator 26, whichcomprises an enclosed tubular chamber, the inside walls of which arecovered with a thin, continuous flowing film of a volatile liquidhydrolyzable titanium compound. The liquid film flows from upperreservoir 28, to lower reservoir 30, from which it is recirculated bymeans of a peristaltic pump 32 to upper reservoir 28. The entireassembly is maintained at temperatures which give a sufficient vaporpressure of the hydrolyzable liquid titanium compound but below thevaporization temperature of the solid nuclei.

Preferably, the temperature of the falling liquid film generator is suchas to give a vapor pressure within the vessel of from about 0.1 to about100 torr. Illustratively, when using titanium (IV) ethoxide as thehydrolyzable compound, a temperature of between 75° and 99° C. ismaintained in the falling liquid film generator; in the case of titanium(IV) isopropoxide, a temperature of between 30° and 60° C.; in the caseof titanium tetrachloride, a temperature of between 0° and 40° C.Because of the inherently high vapor pressure of TiCl₄, the amount ofvapor produced in the inert carrier gas is considerably more than in thecase of titanium (IV) alkoxides.

In general, the gas flow rate through falling liquid film aerosolgenerator 26, is maintained in the laminar region, e.g., correspondingto a Reynolds number less than or equal to 2,000. The linear velocity ofthe falling liquid film should be nearly equal to the linear velocity ofthe flowing gas.

A preferred falling liquid film generator is described in the Journal ofColloid and Interface Science, Vol. 34, page 534, FIG. 1 (1970),incorporated herein by reference. The falling liquid film generator ispreferably modified to contain a head unit of the design shown in FIG.2, comprising carrier gas inlet 26(a), inlet for liquid (from theperistaltic pump) 26(b), liquid head 26(c), falling liquid film 26(d),outlet to constant temperature bath 26(e) and stopper 26(f).

With reference again to FIG. 1, the inert gas stream, comprising thecondensed solid nuclei and the vapor of the hydrolyzable titaniumcompound, passes from falling film generator 26, into first condenser33, at a controlled temperature, where the vapor of the hydrolyzabletitanium compound is cooled to below its liquefaction temperature, andthus condenses upon the solid nuclei in the form of liquid droplets ofuniform size. Illustratively, the condenser is maintained at atemperature of 25° C. for titanium (IV) alkoxides, and at between -6° C.and -30° C. for titanium tetrachloride.

To enhance the uniformity of particle size, the resulting liquid aerosolstream passes from first condenser 33, into heating tube 34, where asecond evaporation of the hydrolyzable titanium compound takes place(the first evaporation having taken place in the falling liquid filmgenerator). Heating tube 34 is maintained at a temperature at leastsufficient to allow complete evaporation of the liquid aerosol. Ingeneral, temperatures corresponding to those employed for thevaporization of the hydrolyzable compound in the aerosol generator, aresuitable. The evaporated liquid aerosol is thereafter recondensed, insecond condenser 36, by means of a cooling line 38.

The evaporation and condensation steps may be repeated again, asdesired. At the end of the second condensation step, or after thedesired number of evaporation and condensation steps, the liquid aerosolof the hydrolyzable titanium compound is ready for hydrolysis andconversion to spherical titanium dioxide.

To obtain the spherical titanium dioxide particles from the liquiddroplets of the hydrolyzable titanium compound, the latter must bebrought into contact with water, and preferably water vapor. Thehydrolysis reaction can be carried out in one or more stages. Thecontact between the liquid droplets of the hydrolyzable titaniumcompound and the water vapor, is achieved by various methods, e.g., bybubbling the liquid aerosol through water, by directing the liquidaerosol over a water surface, or by injecting an inert gas streamsaturated with water vapor into the aerosol stream. Preferably, a gasstream saturated with water vapor is injected into the liquid aerosolstream.

With reference again to FIG. 1, the preferred hydrolysis procedure iscarried out in three stages. In the first stage, stream 42, comprisingan inert gas, e.g., helium, nitrogen, air or the like, which has beensaturated with water vapor, is injected into first hydrolysis manifoldchamber 40, through which the liquid aerosol is permitted to flow. Asuitable manifold chamber is described by McRae et al in the Journal ofColloid and Interface Science, Vol. 53, page 411 (1975), incorporatedherein by reference. The water vapor amount in the carrier gas should bein excess of the stoichiometric amount needed to completely hydrolyzethe titanium compound. In order to avoid too fast an evaporation of theliquid droplets in the aerosol, first hydrolysis manifold chamber 40, ispreferably maintained at the temperature of the condensation of theliquid aerosol.

In the second stage, the partially hydrolyzed liquid aerosol from firstmanifold chamber 40, passes into second hydrolysis manifold chamber 44,maintained at room temperature, where it admixes with a second stream ofwater-saturated inert gas stream 42.

In the third stage, the stream emerging from second manifold chamber 44,comprising the partially hydrolyzed aerosol, excess water vapor from thefirst two stages and reaction products, e.g., HCl, alcohol, and thelike, is heated to a temperature of from about 100° C. to about 250° C.in elongated chamber 46. This completes the hydrolysis.

During the hydrolysis, the liquid droplets of the titanium compoundreact with the water vapor to produce solid spherical titanium dioxideand/or hydroxide particles of substantially uniform size. Theby-products of the hydrolysis will depend on the nature of theparticular hydrolyzable titanium compound employed. For instance, in thecase of titanium tetrachloride as the starting material, hydrogenchloride is released upon hydrolysis. In the case of titanium alkoxidesas the starting material, the corresponding alcohols will be released,e.g., ethanol vapor from titanium ethoxide. The resulting solidparticles of titanium dioxide are extremely pure.

After the hydrolysis, aerosol stream 48, comprising solid, sphericaltitanium dioxide particles and reaction products suspended in thecarrier gas, is treated to separate the titanium dioxide particles.Conventional procedures for the separation of solids from solid/gasaerosols can be used, such as filtration, electrostatic precipitation,thermal gradient deposition by means of a thermopositor, or cyclonecentrifugation.

When titanium alkoxides are used as the starting material, the solidaerosol particles are preferably collected in a thermopositor. Titaniumdioxide particles generated from titanium tetrachloride, on the otherhand, are preferably collected or a Millipore filter, e.g., about 0.22microns pore size, so as to avoid corrosion of the thermopositor by theby-product HCl.

Typical titanium dioxide spherical particles are shown in FIGS. 3 to 6.These are of a substantially uniform shape and size.

If desired, the crystalline structure and the water content of thetitanium dioxide particles can be altered, by subjecing the sphericalparticles to a thermal treatment at temperatures in the range of fromabout 250° to about 1,100° C. This thermal treatment can be carried outbefore the titanium dioxide recovery step (by heating the solid aerosolstream), or after the titanium dioxide recovery step. The formerprocedure is preferred, and it can be performed by direct, or indirect,heating. By means of this procedure, as higher temperatures areemployed, the water content in the spherical titanium dioxide particlesis decreased, and the rutile crystalline content is increased (fromanatase to rutile).

The titanium dioxide particles can be treated to obtain a surface staticcharge, either positive or negative as desired, by forming a suspensionof the particles in water and regulating the pH. The particles of thisinvention normally possess an electrokinetic point of zero charge at apH in the range of from 4.0 to 5.5. Below this pH range, the titaniumdioxide particles are positively charged, and above this pH range, thetitanium dioxide particles are negatively charged.

EFFECT OF CARRIER GAS FLOW RATE AND NUCLEI GENERATOR TEMPERATURE ONPARTICLE SIZE

In general, an increase in the flow rate of the carrier gas for theaerosol results in smaller modal particle diameters. This effect isshown in FIG. 7, for a dibutylphthalate liquid aerosol formed usingheterogeneous nucleation, with NaCl as the material for the solidnuclei, and nuclei generator temperatures set at 590°, 645° and 690° C.,respectively. As is shown, for a given constant temperature, as thecarrier gas flow rate is increased, there is a corresponding decrease inthe modal particle diameter of the condensed liquid particles. Whichultimately results in the recovery of smaller particles of titaniumdioxide, after hydrolysis.

Similarly, an increase in the temperature of the nuclei generatorresults in smaller modal diameters for the liquid particles condensed onthe solid nuclei, as shown in FIG. 7. For instance, at a constant flowrate, as the temperature is increased the modal particle diametercorrespondingly decreases in all cases.

EFFECT OF FALLING LIQUID FILM AEROSOL GENERATOR TEMPERATURE ON PARTICLESIZE

In general, an increase in the temperature in the falling liquid filmaerosol generator, results in larger modal droplet sizes uponcondensation of the vaporized liquid. This is illustrated in FIG. 8 fora dibutylphthalate (DBP) liquid aerosol, using NaCl solid nuclei and anuclei generator temperature of 590° C. Analogous behavior was observedwith liquid titanium compounds in the falling liquid film aerosolgenerator.

EFFECT OF NUCLEI CONCENTRATION ON PARTICLE SIZE

In general, the larger the number of nuclei introduced into a vapor ofthe hydrolyzable titanium compound, the smaller the liquid particlesproduced upon condensation, under otherwise identical conditions.

EXAMPLES

The following examples, which are not intended to be limiting,illustrate this invention.

EXAMPLE 1

Helium, which has been pre-dried over magnesium perchlorate andphosphorus pentoxide and filtered through a Millipore filter having apore size of 0.22 microns, is introduced at a flow rate of 1,100milliliters per minute (ml/min), into a nuclei generator. The nucleigenerator comprises a tubular outer metal casing a sillimanite tubeenclosed within the tubular casing, and a "Vicor" glass boat containingsolid silver chloride (AgCl) as nucleating material. The temperature ofthe furnace is set at 620° C. The helium admixes with vaporized silverchloride, and upon emerging from the nuclei generator, condenses toyield solid nuclei of silver chloride dispersed in the flowing stream ofhelium gas.

The solid nuclei-laden gas is then preheated to 96.5° C. in a coilcondenser, and passed through a falling liquid film aerosol generator atthe same temperature, and a flow rate in the laminar region. The fallingliquid film consists of titanium ethoxide, which is circulated by aperistaltic pump. The linear velocity of the falling liquid film isadjusted to approximately that of the gas flow rate. The titaniumethoxide is vaporized at the temperature of the aerosol generator, 96.5°C., and the titanium ethoxide vapors mix with the flowing stream ofhelium gas and suspended solid nuclei. A stream comprising helium gas,titanium ethoxide vapor and solid AgCl particles (nuclei), emerges fromthe falling liquid film generator. The stream is cooled in a condenserat 25° C. and the titanium ethoxide vapor undergoes a first condensationon the solid AgCl nuclei, to yield a liquid aerosol. The liquid aerosolis heated in a heating tube to complete evaporation of the liquiddroplets, and then recondensed at 25° C. in a second condenser.

The resulting droplets of titanium ethoxide in the aerosol have a narrowsize distribution, showing higher order Tyndall spectra (HOTS), asdemonstrated when light is scattered to show various colors by theparticle when viewed at different angles.

The titanium ethoxide is hydrolyzed as follows: nitrogen gas which hasbeen saturated with water vapor, is admixed with the liquid aerosol byradial injection through a manifold at 25° C. in a first hydrolysischamber. A partly hydrolyzed aerosol flows then into a second manifoldhydrolysis chamber, where it is again admixed with a stream of nitrogengas saturated with water vapor at room temperature. An excess of thewater vapor, about twice that of the stoichiometric quantity necessaryto completely react with the aerosol droplets, is used. To complete thehydrolysis and the conversion of the titanium ethoxide to titaniumdioxide, the aerosol mixture is passed through a tube heated to atemperature of from 100° to 200° C. The resulting solid aerosol,comprising spherical particles of titanium dioxide suspended in heliumgas, is recovered. The titanium dioxide particles have an average(modal) diameter of 0.17 μm, and a width of particle size distribution,σo, of 0.20.

In both the first and second hydrolysis chambers, the aerosol displayshigher order Tyndall spectra (HOTS) indicating a uniform aerosolparticle size. The recovered titanium dioxide, in the form of a powder,is readily dispersible in water by ultrasonication, and the resultingsol also displays higher order Tyndall spectra. Upon inspection byelectron microscopy, the suspended particles are observed to beuniformly spherical, non-aggregated, and of a narrow size distribution.

EXAMPLE 2

The procedure of Example 1 is repeated, except that the temperature ofthe aerosol generator is 90.5° C. The recovered spherical particles oftitanium dioxide have an average (modal) diameter of 0.12 μm, and awidth of particle size distribution σo of 0.20.

EXAMPLE 3

The procedure of Example 1 is repeated, except that a helium flow rateof 150 ml/min, is used. The resulting spherical titanium dioxideparticles have an average (modal) diameter of 0.60 μm, and a widthparticle size distribution σo of 0.16. FIGS. 3 and 4 are scanningelectron micrographs of such titanium dioxide particles which have beensuspended in water and re-dried.

EXAMPLE 4

The procedure of Example 1 is repeated except that a helium flow rate of800 ml/min is used. The resulting spherical titanium dioxide particleshave an average (modal) diameter of 0.22 μm, and the width particle sizedistribution σo of 0.14. FIG. 5 is a transmission electron micrograph ofsuch titanium dioxide particles which have been suspended in water andre-dried.

EXAMPLE 5

The procedure of Example 1 is repeated except that titanium (IV)isopropoxide was used with a helium flow rate of 500 ml/min and thetemperature of the liquid falling film aerosol generator was maintainedat 59° C. FIG. 6 is a scanning electron micrograph of such titaniumdioxide particles which have been suspended in water and re-dried.

EXAMPLE 6

This example illustrates the preparation of titanium dioxide particlesfrom titanium tetrachloride aerosols. The same equipment as in Example 1is used.

A stream of nitrogen, which has been dehumidified over Drierite®, orequivalent, and filtered through a Millipore filter having a pore sizeof 0.22 μm, is introduced at a flow rate of 100 ml/min into a nucleigenerator containing solid silver chloride. The temperature of thegenerator is set at 620° C. The nitrogen stream admixes with vaporizedsilver chloride, and upon emerging from the nitrogen generator,condenses to yield solid AgCl nuclei dispersed in the flowing stream ofnitrogen gas. The solid nuclei-laden gas is then passed through afalling liquid film aerosol generator, maintained at 25° C. by athermostated bath. The falling liquid film consists of titaniumtetrachloride, TiCl₄, which is circulated by a peristaltic pump. Thetitanium tetrachloride is evaporated at a rate of 880 mg/hr, and thevapor is condensed on the AgCl nuclei by cooling to -6° C. in a firstcondenser. The resulting liquid aerosol is then completely evaporated ina heated tube above the vaporization temperature of the titaniumtetrachloride, and recondensed in a condenser at -6° C.

The titanium tetrachloride droplets are hydrolyzed in three stages. Inthe first stage, nitrogen gas saturated with water vapor is admixed withthe liquid aerosol in a manifold chamber at -6° C. In the second stage,nitrogen gas which has been saturated with water vapor, is injected, ina second manifold chamber, into the partly hydrolyzed liquid aerosol ofthe first stage. A gas stream comprising the aerosol particles, hydrogenchloride (which is the product of the hydrolysis of titaniumtetrachloride) and excess water vapor, is then conducted through a glasstube having a length of 40 centimeters, at a temperature of 200° C. Uponemerging from the heating glass tube, the solid aerosol particles oftitanium dioxide are passed through a tubular furnace kept at about 900°C. The titanium dioxide is collected, in the form of a powder, on aMillipore filter having a pore size of 0.22 μm. The yield is about 40%.

A chemical analysis of the recovered powder indicates a purity ofgreater than 99.9%. The remainder consits of 0.02% Ag, and 0.07% Cl.

Upon inspection by electron microscopy, the titanium dioxide particlesare observed to be perfectly spherical. The modal diameter of theparticles is 1.2 μm, and the width of size distribution, σo, is 0.5. Thetitanium dioxide particles are readily dispersible in water byultrasonication. The particles, after separation from water, areobserved to be still perfectly spherical.

Other modifications and variations are possible in the light of theabove description. It is to be understood, therefore, that changes maybe made in the particular embodiments shown herein without departingfrom the scope of the invention, as defined in the appended claims.

We claim:
 1. A method of preparing titanium dioxide, comprising:(A)providing a liquid aerosol comprising discrete liquid droplets of ahydrolyzable titanium (IV) compound; (B) contacting the liquid aerosolwith water vapor in dynamic flow, to hydrolyze the liquid titanium (IV)compound to titanium dioxide in the form of discrete, solid,substantially spherical particles of a substantially uniform shape andsize; and (C) recovering the titanium dioxide.
 2. The method of claim 1wherein the liquid aerosol is made by a process comprising:(a) cooling avapor of the hydrolyzable titanium (IV) compound, in the presence of aninert gas, to a temperature at least sufficient to condense the vaporinto discrete liquid droplets without condensing the inert gas; (b)evaporating the liquid droplets of the titanium (IV) compound from (a);and (c) re-condensing the vapor of the hydrolyzable titanium (IV)compound from (b) to liquid droplets of said compound in a narrowerparticle size distribution, to form the desired liquid aerosol.
 3. Theprocess of claim 2 wherein steps (b) and (c) are each repeated at leastonce.
 4. The process of claim 2 wherein the hydrolyzable titanium (IV)compound and the inert gas are in dynamic flow.
 5. The process of claim2 wherein the vapor of the hydrolizable titanium (IV) compound iscondensed in the presence of the inert gas using homogeneous nucleation.6. The process of claim 2 hwerein the vapor of the hydrolyzable titanium(IV) compound is condensed in the presence of the inert gas usingheterogeneous nucleation.
 7. The process of claim 6 wherein theheterogeneous nucleation comprises condensing the vapor of thehydrolyzable titanium (IV) compound on solid condensation nuclei.
 8. Themethod of claim 1 wherein the hydrolyzable titanium (IV) compound isselected from among liquid titanium (IV) alkoxides and titaniumtetrachloride.
 9. The method of claim 1 wherein the liquid aerosolcontacting with water is effected by a process comprising:(a)introducing a stream of an inert gas saturated with water vapor into aflowing stream of the liquid aerosol, at the temperature of condensationof the aerosol, to form a partial hydrolyzate of the titanium (IV)compound; (b) introducing a second stream of an inert gas saturated withwater vapor into a flowing stream of the partially hydrolyzed liquidaerosol from step (a); and (c) heating the mixture from step (b) at atemperature at least sufficient to completely hydrolyze the titanium(IV) compound to titanium dioxide.
 10. The process of claim 9 whereinthe mixture comprising the water vapor-saturated inert gas and partiallyhydrolyzed liquid aerosol is heated at a temperature in the rangebetween 100° and 250° C.
 11. The method of claim 1 wherein the sphericalparticles of titanium dioxide are heated at a temperature in the rangebetween 250° and 1,100° C.
 12. The method of claim 11 wherein thespherical particles of titanium dioxide are heated at a temperature inthe range between 250° and 1,100° C., before recovery.
 13. The method ofclaim 11 wherein the spherical particles of titanium dioxide are heatedat a temperature in the range between 150° and 1,100° C., afterrecovery.
 14. The method of claim 1 wherein the titanium dioxideparticles have an average (modal) diameter in the range of from about0.05 to about 3 μm.
 15. The method of claim 14 wherein the titaniumdioxide particles have a width of particle size distribution, σo, as lowas 0.1.